(a) Field of the Invention
The present invention relates to an electrochemical element, and particularly, to an electrochemical element with improved energy density comprising multiply-stacked electrochemical cells using multi-component composite films, wherein the multi-component composite films are attached without forming an interface between a support layer film and a porous gellable polymer layer.
(b) Description of the Related Art
There has been growing interest in energy storage technology. The applicable field of the battery has been expanded to cellular phones, camcorders, and notebook computers, and electric vehicles have recently been added to this list. Such expansion has led to increased research and development of batteries with visible outcomes. In this respect, research on electrochemical elements is one of the fields that has been receiving much attention, among which rechargeable batteries are the central field of interest. Recent developments concern designing new batteries and electrodes to improve capacity and specific energy.
Among the secondary batteries being used, lithium ion batteries developed in the early 1990s has become increasingly popular because they have a higher operating voltage and energy density compared to Ni—MH, Ni—Cd, and sulfuric acid-lead batteries that use an aqueous solution electrolyte. These lithium ion batteries, however, have safety problems resulting from the use of organic electrolyte, which causes the batteries to be flammable and explosive. In addition, lithium ion compounds are difficult to manufacture.
Recent lithium ion polymer batteries have overcome such shortcomings of the lithium ion batteries, and are anticipated to become the next-generation batteries. These lithium ion polymer batteries, however, have relatively low capacity compared to lithium ion batteries, and in particular, they have insufficient discharging capacity at low temperatures, and thus they need to be improved.
The capacity of a battery is proportional to the amount of electrode active material used. Thus, it is extremely important to design a cell structure that can be filled with as much electrode material as possible within the limited space of the battery package. The most widely known and used type of cell structure is a jellyroll-shaped structure used in a cylindrical or a prismatic battery. Such a structure is prepared by a process of coating and pressing active electrode material onto a metal foil which is used as a current collector, followed by cutting it into a shape of a strip having a predetermined width and length, separating the negative electrodenegative electrode and positive electrode using the separator film, and then rolling it into a spiral form. Such a jellyroll structure is widely used for manufacturing cylindrical batteries. This structure, however, has a small radius of curvature at the center portion of the spiral, which often results in extreme stresses at the bending surface of the electrode, often causing exfoliation of the electrode. This facilitates the deposition of lithium metal at the center portion of the electrode during the repeated charge and discharge of the battery, which may shorten the lifespan of the battery while degrading its safety.
Generally, the widely known and used method of manufacturing a thin prismatic-shaped battery comprises the aforesaid process of rolling the spiral shaped jellyroll into an oval shape and then compressing it, followed by inserting it into a rectangular container. This method is not free from the aforesaid problems of reduced lifespan and safety, but rather it has increased the problems caused by the decrease in the radius of curvature due to the oval shape. Also, the problem of reduced performance is greater because manufacturing a tight spiral structure is inherently impossible. Furthermore, a discrepancy between the oval shape of the jellyroll and the rectangular shape of the container reduces the rate of utilized volume. This is known to reduce approximately 20% of the weight energy density and 25% of the volume energy density when the container is taken into account. In reality, a prismatic lithium ion battery is reported to have a lower capacity density and specific energy compared to a cylindrical one.
Recently, various patents and technologies proposing to solve the problems of the spiral jellyroll structure and providing cell structures suitable for a prismatic container have been published. These proposals, however, only provide partial solutions to the problems or they cause other problems that are more difficult to solve, so they have not become practical. For example, U.S. Pat. No. 5,552,239 describes a process of first placing and laminating a separator layer or a polymer electrolyte film between the positive electrode and negative electrodenegative electrode, then cutting it into a strip form with a predetermined length and width, followed by gradually folding a cell having an negative electrode/separator layer/positive electrode layered structure into a square form. The inventors of the present invention have tried to replicate this process but they found that it was difficult to manufacture the cells in such a way. The laminated cells were so stiff that they were difficult to fold, and when they were folded by exerting force, a problem arose in the folded area because it was fractured in a manner similar to that of the jellyroll cells.
In a fan-folding method described in U.S. Pat. No. 5,300,373, the pressure and stresses at the inner layer of the abruptly bending portion are transferred to the outer layer and are diverged so that twisting and stretching occur, finally resulting in a “dog bone” shaped cell. Thus, the problems of exfoliation, cracks, and crumbling, encountered in jellyroll structures, also occur frequently. Also, the cells with this structure are inherently prone to snapping, and therefore the possibility of making a practically applicable battery is very low.
Meanwhile, U.S. Pat. No. 5,498,489 attempted to solve and improve such problems in the bending portions. It provides a fundamental way of avoiding exfoliation of the electrodes by leaving out the electrodes at the folding portions and providing connections only through the use of current collectors and separator layers or polymer electrolyte portions, but it is difficult to compose such a cell. Furthermore, too high a volume of the current collectors is used so electrolyte volume is reduced. Thus, the structure is not very practical because it has many inefficient factors.
Electrolytes are classified as liquid electrolyte and solid electrolyte. Liquid electrolyte comprises a salt dissolved and dissociated in an organic solvent, and it has high ionic conductivity. Liquid electrolyte is generally used together with a polymer separator, e.g. a polymer film such as a polyolefin with pores that has ionic conductivity because of liquid electrolyte in the pores. The ionic conductivity varies depending on the porosity of the polymer separator, and the polyolefin separator generally has an ionic conductivity of about 1 mS/cm.
But the liquid electrolyte may leak out of the polymer separator due to its high fluidity. In addition, the liquid electrolyte cannot provide adhesion between an electrode and a separator, and thereby the battery is structured with an interface between them. In spite of these disadvantages, it has an advantage of high mechanical strength due to its high crystallinity, thus it neither over-swells nor decomposes.
On the other hand, the solid electrolyte has an ionic conductivity that is insufficient to be used in a battery at room temperature. In order to improve the ionic conductivity of the solid electrolyte, a gellable polymer electrolyte has been suggested, in which liquid electrolyte comprising a salt dissolved in an organic solvent is impregnated in a solid polymer electrolyte, e.g. a hybrid-type electrolyte as disclosed in U.S. Pat. No. 5,418,091, available from Bellcore Co. However, when the gellable polymer electrolyte is used for an electrolyte of a battery, there are problems in battery assembly due to its low mechanical strength, and the polymer electrolyte may be over-swelled, its thickness may increase, and energy density may decrease due to a decrease in the density of the polymer electrolyte, even though the polymer electrolyte has a thickness of greater than 50 μm in order to insulate between electrodes and to obtain sufficient mechanical strength in a battery. Furthermore, since a plasticizer having a low molecular weight that is harmful to the environment is used, and an extraction process thereof is further required, it is problematic to mass-produce a battery with the solid electrolyte.
The polymer electrolyte requires electrochemical stability in working voltage, and thermal and chemical stability. Preferably, it has an ionic conductivity of more than 1 mS/cm at room temperature, a wet-out rate that is superior to that of non-aqueous electrolyte, and high chemical-resistance. In addition, it is preferable that the polymer electrolyte adhesion is sufficient to decrease the interfacial resistance between the electrolyte and electrodes during battery assembly, and that it has enough mechanical strength during battery assembly. However, it is known that when the ionic conductivity increases, the mechanical strength deteriorates, and vice versa.
To increase both the ionic conductivity and the mechanical strength, it is disclosed that a porous polymer layer and a gellable multi-layer film are used for a separator in U.S. Pat. Nos. 5,639,573, 5,716,421, 5,631,103, and 5,849,443, and in European Patent Application No. 0 933 824 A2. The porous polymer layer comprises a material that is resistant to swelling due to restrictive absorption of liquid electrolyte, and the exemplary materials include polyethylene, polypropylene, polytetrafluoroethylene, polyethylene terephthalate, polybutyleneterephthalate, and polyethylenenaphthalate, and a multi-layer film or film blended thereof. The gellable polymer comprises a self-gellable and self-swellable material when it is contacted with liquid electrolyte, and the exemplary materials includes polyvinylidenefluoride, polyurethane, polyethyleneoxide, polyacrylonitrile, polymethylmethacrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidinone, and polytetraethylene glycol diacrylate, and a copolymer thereof.
When the aforementioned separator is used, the mechanical properties can be improved, but the polymer electrolyte has ionic conductivity that is lower than those of the porous polymer and the liquid electrolyte dissolved therein, resulting from the ionic conductivity-resistance of the gellable polymer. U.S. Pat. Nos. 5,631,103 and 5,849,433 disclose that a plasticizer having a low molecular weight, such as dibutyl phthalate, is used in order to enhance the ionic conductivity of the separator. However, the plasticizer is harmful to the environment, and it makes mass production of a battery difficult.
In addition, a multi-layer film prepared by the aforementioned method has a dense gellable polymer layer having no pores, its ionic conductivity-resistance increases undesirably, and an interfacial adhesion strength between the porous polymer layer and the gellable polymer layer weakens.
Further, although various separator layers or separator films are used in a battery made with many stacked cells, it is still required to provide a separator film or separator layer having a high ion conductivity, good interface contact characteristics between electrode and electrolyte, and high mechanical strength.